The invention relates to corrosion inhibitors, and, more particularly, to compositions of ammonium carbonate and ammonium bicarbonate that are suitable to inhibit the corrosion of ferrous materials exposed to chlorides.
There has been a tremendous demand for a chemical which will inhibit the corrosion of steel by chlorides such as sodium chloride, and much research has been conducted in an attempt to discover such a chemical.
Steel is commonly exposed to chloride ions in unavoidable open and enclosed environments. Structures containing steel are exposed to salt water environments in and around oceans or other bodies of salt water and are seriously corroded by chlorides present in the water. In an enclosed environment, calcium chloride is added to concrete to accelerate its rate of setting. These chloride additions have, however, caused the reinforcing steel in the concrete to corrode.
Another significant source of chloride corrosion has been deicing compositions applied to the paved surfaces of roads and highways. These compositions have been splashed onto the steel contained in the highway structures as well as onto metal vehicle bodies. The corrosion thus produced has been quite visible in the form of corroded structures and vehicle bodies. Another well-known and highly visible chloride-related damage to highways has been potholes. Potholes form as the corrosion deposits caused by the action of the chlorides on the reinforcing steel expand and create various stresses on the concrete which subsequently causes large pieces of concrete to fracture and separate from the roads and highways.
Exemplary illustrations of the tremendous amount of research which has been performed in an effort to learn how to inhibit the corrosion of steel and protect it from the effects of chlorides, are found in the following publications: Mozer, Bianchini and Kesler, "Corrosion of Reinforcing bars In Concrete," Journal of the American Concrete Institute, August 1965; "Concrete Information" by the Portland Cement Association (1968); John G. Hendrickson, "Corrosion of Steel in Concrete," Portland Cement Association (1968); and Bailey Tremper, "Corrosion of Reinforcing Steel," American Concrete Institute. These publications in turn list dozens of additional publications focusing on the same subject.
Numerous attempts have been made to identify specific corrosion inhibitors capable of preventing the corrosion of steel. Inhibitors are chemical substances which effectively decrease the corrosion rate in an environment. The corrosion rate may be reduced by such a large extent by inhibitors that the corrosion reaction is effectively stopped in some instances. For example, Gouda and Monfore describe in their publication entitled, "A Rapid Method for Studying Corrosion Inhibition of Steel in Concrete," the testing of some of the well known corrosion inhibitors, such as sodium nitrite, potassium chromate and sodium benzoate. The authors found that to inhibit the corrosion of a 2% addition of calcium chloride to concrete, additions of 1%-2% of sodium nitrite and 2%-4% of potassium chromate were needed. Because sodium benzoate is precipitated by calcium chloride, the authors tested sodium benzoate against 1.6% of sodium chloride and determined that it requires 4%-6% of sodium benzoate to inhibit corrosion in the presence of this amount of sodium chloride.
Because of their undesirable side effects, however, most corrosion inhibitors must be carefully administered. For example, the chromates and other effective chromate salts are highly toxic and will produce ulcers on any skin they contact. As a further example, the publication entitled "Effect of Various Substances on Concrete and Protective Treatments, Where Required", Portland Cement Association (1968), explains that sodium nitrite causes slow disintegration of concrete. To date, a satisfactory corrosion inhibitor for chlorides has not been found.
The above-described inhibitors are anodic inhibitors. According to Gouda and Monfore, the inhibitors can form "sparingly soluble iron salts or gamma Fe.sub.2 O.sub.3 films on the anodic areas, thus preventing ferrous ion from passing through into solution." These salts form according to the classical understanding that the corrosion of steel is an electrochemical process. At the steel anode, iron goes into solution and forms a ferrous ion and releases two electrons: Fe.fwdarw.Fe.sup.++ +2e.sup.-. At the steel cathode, the two electrons produced at the anode react with two hydrogen ions to form a hydrogen film: 2H.sup.+ +2e.sup.+ .fwdarw.H.sub.2. In instances when the supply of oxygen is limited and the pH is relatively high, an anodic film forms on the steel and prevents further corrosion. When chloride ions are present, however, the protective anodic films are removed by forming soluble chloride compounds and the steel is exposed to further electrochemical attack by the chlorides.
To comprehend the severity of the problem of corrosion, in 1976 the Environmental Protection Agency estimated that using sodium chloride or calcium chloride as deicing compounds costs the U.S. about five billion dollars per year. Probably a significant percentage of this amount can be attributed to the corrosion of steel. Despite the damage salt has produced, it has proven to be a good deicing chemical and has undoubtedly saved thousands of lives and injuries, which would otherwise have resulted from ice-covered roads.
Although salt has been successful for the purpose of deicing roads and highways, it has caused the corrosion of many thousands of highway structures. Consequently, a large amount of money will continue to be needed to repair or replace these damaged structures until a suitable corrosion inhibitor is available for large-scale application.
Thus, there has been an urgent need for a composition which can be applied directly to steel or to structures containing steel so as to inhibit the corrosion already started by deicing chemicals containing chlorides. Such a composition would have the potential to save billions of dollars which would otherwise be spent to rebuild the presently corroding highway structures. Once this corrosion is arrested, the saved repair money can be used to purchase improved deicers having the desirable deicing qualities of salt but lacking its damaging corrosive side effects.